Sigma in Germany was very good overall - fast & relatively low priced for a non guarantee issue (replacement of scratched front element of a 1200$ lens for about 190$ incl. shipping and complete check and realignment)

I had two occurrences where I needed Sigma service. Both were on my 70-200. The first one was some sort of dirt which had accumulated behind the front element. I would say it was some sort of fungi since it was a prefect ring forming at about . Sigma cleaned the lens for free but added the disclaimer that this was not a warranty issue and that the free service was voluntary from their side. This time it was send through the store and I had my lens back in about 5 working days. Second occurrence was a broken AF at the outer edge of my warranty (Three weeks later or so and I would have had to find out just how flexible they really are). I thought to be clever and actually brought it in myself since their German service station is rather close to my work. turns out that this was maybe not such a good idea. The lens was repaired for free, but this time it took them something like 3-4 weeks. Though to be fair their service station is not really equipped to receive equipment for repair, so it might have gotten lost in the shuffle a bit there.Both repairs were flawless and both times the lens was very clean and free of any visible dust inside. (Which was not the case before the repair, though it never affected image quality as far I as could tell) The lens held up after that, at least until it was stolen

AdamJ

My Sigma 12-24mm II recently suffered an electrical failure. It was returned via the dealer to Sigma UK under warranty. I received the repaired lens back just under four weeks later in the same pristine condition as it was before. I'm assuming the AF was calibrated as a matter of course, though I can't tell since the focusing was dead-on before and it still is.